Recently, it became problematic for an automotive industry to develop a vehicle structure that is capable of eliminating or significantly reducing an injury for a passenger at a time of a collision. The vehicle structure having such crashworthiness can be obtained such that an impact energy at the time of a collision is absorbed by structural members other than a passenger section, and a deformation of the passenger section is minimized to thereby securing a survival space. Namely, it is important to make the structural members absorb the impact energy.
A main structural member absorbing the impact energy at a full-lap crash or an offset crash of an automobile can be a front side member. In the front side member, the cross section of the member can be closed by a spot welding after the member is formed by a press forming process, etc. Generally, this front side member can be collapsed to thereby absorb the impact energy. It may be beneficial to stabilize a buckling mode and possibly prevent and reduce its bending and/or a fracture halfway, e.g., to improve the absorption of the impact energy.
There may exist certain problems such as a fracture which may occur from a welded point at the time of buckling to cause an unstable buckling mode and the absorption of the impact energy deteriorates, if a spot welding interval, a nugget diameter, and a welding condition are not optimized to stabilize the buckling as for the above-stated spot welding of the member.
Conventionally, a condition has been reviewed to solve a problem, in which, for example, a member is buckled stably without fracturing at a welded point by manufacturing members experimentally while changing spot welding intervals variously, and performing a crash test, as described in Expository Papers No. 9705 JSAE SYMPOSIUM “New Forming Technique of Vehicle Structure”.
However, in the above-described method, trial and error should be used in which experimental products are manufactured by each automobile, or by each member to perform the test. Therefore, a problem may exist such that a manufacturing cost becomes high and a long time for design.
In addition, as described in Japanese Patent Application Laid-Open No. Hei 6-182561, a fracture prevention structure of a welded portion of a floor panel where a load is applied is proposed, but it is a structure only for the floor panel. Therefore, it still required trial and error by the experimental products to obtain a spot welding method in which fracture at the welding points is prevented in every impact absorbing member, and the impact energy is absorbed by the stable buckling.
Further, Japanese Patent Application Laid-Open No. 2002-31627 describes an optimization of the spot welding interval. However, it provides only a simple index as for individual spot welding strength, and may not provide an accurate prediction for the fracture as it is. Therefore, a problem exists in which a design based on an accurate fracture prediction for the spot welded portion may be very difficult or even impossible.
As the index of the strength at the spot welded portion, a shear tension test and a cross tension test as described in Specifications JIS Z3136 and JIS Z3137 are representative thereof. The examples under various test modes (assuming various load states other than the above) in general describe that a shear tension test value is treated as a shear strength of the welded portion, and a cross tension test value is treated as a peeling strength of the welded portion by two kinds of tests prescribed in JIS.
However, the shear strength and the peeling strength of the spot welding obtained by the test can come under a structural influence, such as a width. Therefore, the test value cannot help being corrected and estimated from various points of view in an actual member. In a system making an optimal design by a collision simulation of an automobile on a computer which is advancing rapidly in recent years, an estimation accuracy thereof may not be adequate, and a reliability of the optimal design for a collision safety has likely been deteriorated.